Thermoplastic urethane resins have superior mechanical strength and wear resistance on account of which these resins find many uses such as for hoses, belts, coatings for electric wires, pipes, shoe soles and various other moldings. However, urethane resins have easily hydrolyzable ester urethane bonds and are not suitable for use in areas where they are exposed to moisture for a prolonged period of time or in applications where steam or hot water is used. Efforts are being made to improve the resistance of water of urethane resins by using polyols having ether bonds [e.g., poly(1,4-oxybutylene)glycol], or caprolactam-based polyols (e.g., .epsilon.-lactone ester polymers instead of the aliphatic esters having easily hydrolyzable ester bonds. However, the inherent problem of hydrolysis still exists in urethane resins. A further problem with urethane resins is that they melt at temperatures of 180.degree. C. or higher and cannot be used in applications where they are exposed to high temperatures as 150.degree. C. or above, such as in the operation of dipping electric wires in a solder bath, without causing deformation of the resin coat.
With the rapid increase in the use of NC controlled machine tools, the field in which materials having high mechanical strength (e.g., high wear resistance) are used is expanding and the development of urethane resins having high resistance to heat and hot water has been sought.
From the view point of preventing fire and other disasters, the requirements for flame retardancy are becoming increasingly strict and there is a great need to offer a molding of a urethane resin composition, such as a coated electric wire, that is flame-retardant and exhibits superior resistance to water and heat.
One conventional method for improving the resistance to heat of high-molecular weight materials is to crosslink individual polymer molecules as is frequently practiced with polyethylenes. Crosslinking is commonly achieved by chemical crosslinking with organic peroxides, by radiation crosslinking with electron beams or gamma rays, or by water crosslinking with a reactive silane. However, chemical crosslinking and water crosslinking are unsuitable for thermoplastic urethane resins because the temperature for molding is at least 180.degree. C., which is higher than the decomposition temperature of organic peroxides, and the addition of a reactive silane is uncontrollable.
The common technique for effecting radiation crosslinking is to add reactive polyfunctional monomers, to thereby cause accelerated crosslinking. It is generally held that a higher crosslinking efficiency is attained by polyfunctional monomers that have many functional groups and have a low molecular weight of monomers per functional group. Polyfunctional groups that are commonly employed include diacrylates such as diethylene glycol diacrylate; dimethacrylates such as ethylene glycol dimethacrylate; triacrylates such as trimethylolethane triacrylate and trimethylolpropane triacrylate; trimethacrylates such as trimethylolethane trimethacrylate and trimethylolpropane trimethacrylate; as well as triallyl cyanurate, triallyl isocyanurate, diallyl phthalate, trimethylmethacryl isocyanurate, trimethylacryl cyanurate, trimethylacryl isocyanurate and triacrylformal.
The present inventors added these polyfunctional monomers to thermoplastic urethane resins and studied their effectiveness in radiation crosslinking. To their great surprise, the urethane resin compositions having incorporated therein polyfunctional monomers other than trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, and triacrylformal experienced total deformation in a thermal deformation test conducted at 180.degree. C.
It is well known by, for example, U.S. Pat. No. 3,624,045 that to the thermoplastic urethane resin, N,N'-methylene-bis-acrylamide or N,N'-hexamethylene-bis-maleimide is added as a polyfunctional monomer, followed by effecting the radiation crosslinking. Although these polyfunctional monomers are an effective crosslinking monomer, urethane resins crosslinked with such a polyfunctional monomer are greater in terms of reduction in strength in hot water at 100.degree. C. than those crosslinked with trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, or triacrylformal and, therefore, the use of such polyfunctional monomers is not suited for attaining the object of the present invention.
Several of the polyfunctional monomers tested had the following molecular weights per functional group: 112.6 for trimethylolpropane trimethacrylate (molecular weight: 338), 98.7 for trimethylolpropane triacrylate (molecular weight: 296), 83 for triacrylformal (molecular weight: 249), and 83 for each of triallyl cyanurate and triallyl isocyanurate (molecular weight: 249). In consideration of the generally held view about the reltionship between the number of moles of a functional group and the degree of crosslinking, triallyl cyanurate would be expected to achieve a higher degree of crosslinking than trimethylolpropane trimethacrylate added in the same amount. Accordingly, there was much reason to expect that urethane resin compositions having incorporated therein triallyl cyanurate and triallyl isocyanurate would experience less deformation at 180.degree. C., than those tested after incorporation of trimethylolpropane trimethacrylate, trimethylolpropane triacrylate and triacrylformal. Curiously enough, however, the improvement in resistance to thermal deformation which was attainable by radiation crosslinking was observed only with the urethane resin compositions having incorporated therein trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, or triacrylformal.
A hot water test which was subsequently conducted at 100.degree. C. showed that the urethane resin compositions that had been radiation crosslinked suffered from a smaller decrease in tensile strength than non-crosslinked urethane resins.